Inkjet printing is a non-impact method for producing images by depositing ink droplets on an image-recording element in response to digital signals. Various methods have been used to control the deposition of the ink droplets on the image-recording element. In one process, known as a continuous inkjet printing process, a continuous stream of droplets is charged and deflected in an image-wise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as a drop-on-demand inkjet printing process, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Inkjet printers have found broad applications across markets, including industrial labeling, short run printing, fleet graphics (e.g., those used on large advertisement signs), and desktop document and pictorial imaging.
Special challenges exist in formulating inks suitable for use in inkjet printers because inkjet printing applications often demand many different properties from the inks employed in the application. Examples of specific properties required of suitable inks include the ability to adhere to specific types of substrate materials, the ability to adhere to a broad range of substrate materials, the ability to cure or dry within a certain allotment of time, and/or the ability to withstand various environmental conditions. In addition, ink formulations should also be compatible with printing equipment and methods.
In order to meet these special challenges, prior art inkjet inks have often included chemicals that are toxic or environmentally hazardous. For example, conventional ink formulations, especially those for commercial use, often rely on a number of ingredients whose environmental and human health profiles can be improved. Foremost among them are solvents, many of which are problematic: the ethylene glycol ethers raise many human health concerns, including potential blood, kidney, reproductive, and central nervous system effects. Benzyl alcohol poses both human health and environmental concerns, especially for their potential toxicity to aquatic organisms. Solvents having low flash point or high volatility pose environmental hazards. Some common surfactants, like the nonylphenol ethoxylates, biodegrade to compounds that are more aquatically toxic than the parent, persist in the environment, and may disrupt endocrine systems (responsible for metabolism, reproduction, and growth). These toxic chemicals threaten the safety and health of those people that handle the chemicals, and often leach or migrate into groundwater or other portions of the environment. Also, ventilation of the printing and ink manufacturing equipment in order to mitigate the health risks associated with using these chemicals increases production costs.
A need exists for inkjet ink formulations that overcome or minimize these problems, yet adhere to a relatively broad range of substrate materials.